Researchers have found that there is a possible connection between large seismic shocks and the reversal of tectonic plates.
Scientists have been studying two of the largest earthquakes that have recently occurred, namely: i) the M 9.0 Tohoku earthquake which struck offshore Japan in 2011 and resulted in a massive tsunami devastating a Nuclear Power Plant in Fukushima and, ii) the M 8.8 Chile earthquake which occurred offshore Chile also triggering a tsunami and causing extended damage. What they discovered is that those “...earthquakes were preceded by reversals of 4–8 millimeters in surface displacement that lasted several months and spanned thousands of kilometers”.
Dr. Jonathan Bedford, lead author of the study and a researcher at GFZ German Research Centre for Geosciences, explains that this movement may seem small but it is actually important considering that plates relative motions are just a few centimeters annually.
Both earthquakes occurred on subduction zones, which are defined as the regions where two tectonic plates meet and one slides underneath the other. During this process, long and deep depressions, known as trenches, are created.
In both cases the oceanic plates are sliding underneath the continental plates, thus, the continental plates are moving away from the trench. However, the results showed that this movement reversed multiple times before the said large earthquake events.
The team managed to measure the relative movement of already installed ground stations by manipulating data from the Global Navigation Satellite System (GNSS). In particular, the distance between a given ground station and a group of GNSS satellites is constantly recorded. Assuming that the positions of the satellites are defined, scientists can derive the actual movement of points established on the Earth’s surface. The system was highly effective both in Japan and Chile where there is a wide and extensive network of such stations.
Their analyses included 5-year data before each tremblor. The findings showed that reversal movements occurred in both Japan and Chile earthquakes just 5 and 7 months before the major incidents, respectively. Scientists also found out that this pattern is not localized and extends for thousands of kilometers along the boundaries of the plates.
According to researchers, the findings of the present study should be interpreted wisely. It is not certain that every large earthquake will be preceded by a similar pattern. What is really important is to understand the complex dynamic behavior of subduction zones. "It is a common assumption that deeper subduction proceeds at a fairly constant speed in between large earthquakes. Our study shows that this assumption is an oversimplification. In fact, its variability might be a key factor in understanding how the largest earthquakes nucleate," Dr. Bedford, explained.
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